Reduced particle settling speed in turbulence

We study the settling of finite-size rigid spheres in sustained homogeneous isotropic turbulence (HIT) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We study semi-dilute suspensions at different Galileo numbers, Ga. The Galileo number is the ratio between buoyancy and viscous forces, and is here varied via the solid-to-fluid density ratio. The focus is on particles that are slightly heavier than the fluid. We find that in HIT, the mean settling speed is less than that in quiescent fluid; in particular, it reduces by 6%-60% with respect to the terminal velocity of an isolated sphere in quiescent fluid as the ratio between the latter and the turbulent velocity fluctuations is decreased. Analysing the fluid-particle relative motion, we find that the mean settling speed is progressively reduced while reducing the density ratio due to the increase of the vertical drag induced by the particle cross-flow velocity. Unsteady effects contribute to the mean overall drag by about 6%-10%. The probability density functions of particle velocities and accelerations reveal that these are closely related to the features of the turbulent flow. The particle mean-square displacement in the settling direction is found to be similar for all Ga if time is scaled by (2a)/u' (where 2a is the particle diameter and u' is the turbulence velocity root mean square).Comment: Accepted for publication in Journal of Fluid Mechanic

Pegged exchange rate regimes -- a trap?

This paper studies the empirical and theoretical association between the duration of a pegged exchange rate and the cost experienced upon exiting the regime. We confirm empirically that exits from pegged exchange rate regimes during the past two decades have often been accompanied by crises, the cost of which increases with the duration of the peg before the crisis. We explain these observations in a framework in which the exchange rate peg is used as a commitment mechanism to achieve inflation stability, but multiple equilibria are possible. We show that there are ex ante large gains from choosing a more conservative not only in order to mitigate the inflation bias from the well-known time inconsistency problem, but also to steer the economy away from the high inflation equilibria. These gains, however, come at a cost in the form of the monetary authority's lesser responsiveness to output shocks. In these circumstances, using a pegged exchange rate as an anti-inflation commitment device can create a "trap" whereby the regime initially confers gains in anti-inflation credibility, but ultimately results in an exit occasioned by a big enough adverse real shock that creates large welfare losses to the economy. We also show that the more conservative is the regime in place and the larger is the cost of regime change, the longer will be the average spell of the fixed exchange rate regime, and the greater the output contraction at the time of a regime change.Foreign exchange rates ; Monetary policy

Too far ahead of its time: Barclays, Burroughs and real-time banking

The historiography of computing has until now considered real-time computing in banking as predicated on the possibilities of networked ATMs in the 1970s. This article reveals a different story. It exposes the failed bid by Barclays and Burroughs to make real time a reality for British banking in the 1960s

'Too far ahead of its time': Britain, Burroughs and real-time banking in the 1960s

In 1969, the popular British television programme, Tomorrow's World, featured an item that predicted point of sale terminals in every high street shop ushering in the country's computerised cashless economy. The basis for the show's prediction was a succession of ambitious projects initiated by the British banks, each with the aim of introducing a new real-time computer banking system to its network of branches by 15 February 1971. The banks, threatened by state-sponsored competition, inspired by the success of SABRE, American Airlines' real-time airline reservations system, and pressured by forthcoming decimalisation, all chose 'D-Day' as their immovable deadline. And, in each case, US computer manufacturer, Burroughs, promised a B8500 central super computer linked to a nationwide network of TC500 intelligent terminal satellites. Perhaps unsurprisingly for Tomorrow's World, the programme's predicted coming of the cashless society was wildly optimistic. But so too, it turned out, were the plans of the banks. Real-time banking in Britain never materialised in the 1970s, let alone by February 1971, as one by one the banks abandoned their plans. In this paper, I revisit the case of Burroughs and Barclays Bank. Blending oral testimonies with archival sources, I explore a consumer perspective of coterminous computing labour as the two companies set about making the idea of real-time banking a reality. I reveal how a community of practice made up of Barclays' computer programmers and Burroughs' engineers was able to transgress established business boundaries in pursuit of a technical ideal, only to eventually become architect of its own fate. The co-construction and 'interpretive flexibility' of this technological failure is considered in light of the existing literature, with particular attention given to the attribution of blame. In this case, where there was attribution, it was judged to have lain with the technology, which was simply regarded as 'too far ahead of its time.ï¿